One to three days before you'll be cooking, combine the salt, brown sugar, and whatever other spices you'd like and carefully rub it into the pork picnic. Set the pork onto a cooling rack that fits onto a rimmed sheet tray and put it back into the fridge, giving it 24 hours uncovered. (If you plan to go longer than a day in the fridge, cover the meat until 24 hours before you plan to roast it, then unwrap)
Preheat the oven to 300° F. Put pork in a big pot with a cup of stock, making sure the fatty part of the pork is facing upwards, Cook for 6 to 10 hours, until the pork is fork tender.
As soon as the pork is cool enough to handle, use two forks to pull the meat from the bone. Do this on a sheet tray so any juices that run get blended back into the pulled pork. The warmer the pork, the easier it is to shred.
For the cabbage slaw and assembly:
Combine the vinegars, Dijon, and salt in a mixing bowl and whisk until the salt is dissolved.
Add the oil and whisk until the dressing is emulsified.
Place the shredded fennel and cabbage in the bowl on top of the dressing, and toss to combine.
Add the fennel fronds and toss again just before serving.
Assemble the sandwiches (either 6 buns or fewer subs): a layer of pork, a layer of slaw, and a sprinkling of pickled jalapeños.